The present invention relates to the field of drilling, or more generally the field of operations requiring circulation of a fluid in a drilled well, said fluid being in contact with the porous, permeable rock walls. During these operations, the circulating fluid penetrates into the thickness of the wall to a greater or lesser degree depending on the effectiveness of the filtrate-reducing additives that can be added to the fluid. It is known that the xe2x80x9cfiltratexe2x80x9d (volume of fluid penetrating into the rock) can cause the well wall to clog to varying degrees. If the well passes through a hydrocarbon-producing area, productivity can be seriously impaired. The present invention proposes a fluid and a method for drilling a well, or intervening in a drainhole intended for starting production in a hydrocarbon-producing formation while favoring the flow of these hydrocarbons to the well.
The fluid used when drilling a well circulates at right angles to the formation whose pressure is higher than the pressure of the formation. This enables inflow phenomena to be limited and the stability of the well walls to be controlled. Filtration phenomena occur, forming a filtration cake. Depending on the circulation conditions (whether or not mud is circulating in the well), an external and internal cake, or just an internal cake, is formed in the so-called xe2x80x9cspurtxe2x80x9d period. This very-low-permeability cake protects the formation, and in particular the reservoir rock, from invasion by the constituents of the drilling mud. However, recent studies (A. Audibert, J-F Argillier, H. Ladva, Way P., Hove A. xe2x80x9cRole of Polymers in Formation Damagexe2x80x9d SPE 54767 presented to the SPE Formation Damage, The Hague, 1999) show that a zone is formed, beyond the internal cake, invaded by certain mud components. In particular, in the case of an oil reservoir drilled with water-containing mud, the reservoir is seen to be invaded beyond the internal cake by a fraction of the polymers contained in the mud. These hydrophilic polymers are not held back in the cake and become adsorbed in the pores of the rock, thus reducing the permeability of the rock to oil. This phenomenon is more significant in the vicinity of the well, and causes water saturation to increase near the well.
The invention applies advantageously to water-containing mud formulations proposed for crossing reservoirs, more generally known as xe2x80x9cdrill-in fluidsxe2x80x9d (described for example in the document xe2x80x9cDrill-in Fluids Control Formation Damagexe2x80x9d by Halliday W. S., World Oil, December 1994,43-47).
The well fluids are complex fluids whose main functions are to participate in cleaning the well during drilling or after it has been drilled (completion fluids), apply a sufficient hydraulic pressure to ensure the safety of the well, stabilize the well walls, lubricate the casing strings or strings of rods, or enable equipment to be set up at the bottom or the production process to be implemented (completion fluids). To bring about these functions, the basic fluid, water in this case, has specific products added to it to regulate the rheology, density, cleaning ability, and filtrate control, for example. No well fluid additive that has a direct effect on the saturation of the rock in contact with the fluid is known.
Thus, the present invention relates to a drilling or intervention method in which fluid is made to circulate in a well drilled into a hydrocarbon reservoir rock. The fluid is water-based and includes all the additives required for the functions of a drilling fluid and/or interventions, and a sufficient quantity, 10 wt. % as a maximum, preferably less than 5%, of an ester-based additive composition obtained by glycerolysis, by direct esterification, or by transesterification from a grease (or an oil) and an alcohol.
Such additives are classically used in secondary or tertiary recovery when the reservoir is flushed. These recovery phases take place when a reservoir is produced after the primary recovery phases. These additives are added to a polymer solution or as a preflush to reduce the water-oil interfacial tension and improve eventual recovery of oil by flushing. Surprisingly, the use of such additives in drilling fluids and/or interventions reduces and prevents damage to the reservoir during the drilling and/or intervention phases in wells drilled in hydrocarbon reservoir rocks. Of course, the nature of these additives is determined based on the particular nature of the water-based well fluids that contain other additives with which the additive according to the invention must be compatible, i.e. not impede the functions of other additives or components of the well fluid.
The ester of the present invention is obtained by glycerolysis, by direct esterification, or by transesterification from a grease (or an oil) and an alcohol. All vegetable or animal greases may be suitable if their chain length is between C6 and C22, preferably between C6 and C12.
The grease (or oil) must also include triglycerides, fatty acids, and methyl esters. The entire castor oil equivalent range must also be included.
The alcohols of the present invention include diols, triols, polyols, and mixed polyalkoxides.
The following may be cited in particular:
glycerol: HOCH2xe2x80x94CHOHxe2x80x94CH2OH;
polyglycerol
HOCH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94(OCH2xe2x80x94CHOHxe2x80x94CH2)nxe2x80x94OCH2xe2x80x94CHOHxe2x80x94CH2OH
where n is between 1 and 8, preferably between 1 and 4;
ethylene glycol; HOCH2CH2OH;
polyethylene glycol: H(OCH2CH2)nOH,
xe2x80x83where n is between 2 and 30, preferably between 2 and 12,
propylene glycol: CH3xe2x80x94CHOHxe2x80x94CH2OH;
polypropylene glycol: H(OCH)CH3)CH2)nOH,
xe2x80x83where n is between 2 and 30, preferably between 2 and 12,
polyethylene glycol-polypropylene glycol mixtures.
The additive composition according to the invention combines the various properties required for its maximum effectiveness, particularly with regard to the change in wettability of the rock.
In other words, the additive composition has the following features:
good dispersion in water of the well fluid, i.e. sufficient water solubility, even if not total solubility,
very good compatibility with the water-based fluid, i.e. does not interfere with the properties required for the mud, provided by other compounds or additives,
passes directly into the filtrate, i.e. is not held back or blocked in the cake that may form on the well wall,
does not form an emulsion with the reservoir oil,
and adsorbs sufficiently on the rock.
The additive according to the invention can be used in a concentrated form (for example approximately 1 g/l) or diluted in a solvent oil (a vegetable or animal oil described above).